Beneficial Effects of Boron Addition into γ-TiAl Alloys

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The beneficial effects of boron addition on microstructure transformations and
mechanical properties of γ-TiAl alloys were investigated. Two growth mechanisms of boride (TiB2)
in γ-TiAl alloy were confirmed, the curved flaky borides are products of irregular eutectic reaction
growing coupled with matrix, while some faceted blocky borides in boron-rich alloy are primary
TiB2 phase growing directly in melt. The core of flaky TiB2 is ultra-fine B2 phase and there has an
orientation relationship [1210] TiB2//[001]B2, (1010) //(010)B2. In addition to the well-known
grain refinement effect, boron addition can suppress the formation of metastable feathery and
Widmastätten structure and broadens cooling-rate-range for the formation of fully lamellar structure,
consequently, it improves thermal stability of the lamellar structure and accordingly prolongs the
creep rupture life significantly. Another beneficial effect of boron addition is that boride can
restrain discontinuous coarsening on lamellar grain boundary by pinning action and accelerates
recrystallization of γ grain by introducing TiB2/matrix interfaces as nuclear sites during
homogeneous treatment at 1150°C. Therefore, compared with boron-free alloy more homogeneous
and refined near γ microstructure can be obtained in boron modified alloy.

Abstract: In this paper, effects of strontium on Mg alloys and preparation technology of Mg-Sr and Mg-Sr-Al master alloys were summarized respectively. Prospects and applications of Sr-Mg master alloys were analyzed too. The results showed that thermodynamics calculation of reaction between SrO and Mg(l) showed that molten Mg can reduce SrO, in which excess of molten. The microstructure of hypoeutectic Mg-Sr binary alloys is composed of primary Mg matrix, and laminary eutectic phase(Mg-Mg17Sr2),but the microstructure of Mg-Sr-Al alloy consists of
plate-like primary Al4Sr, laminary eutectic phase (Mg-Mg17Sr2 or Mg-Mg17Al12 or
Mg-Mg17Sr2-Mg17Al12) embedded in primary dendrites of magnesium matrix. Additive strontium in Mg alloys can refine it grains, reduce degree of porosity, improve mechanical and thermal properties, therefore, Mg-Sr master alloys can be served widely as an additive for modification or as a constituent of Mg alloys, Al alloys, Zn alloys, etc.

Abstract: Inconel 718(Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) nickel-base superalloy strengthened
mainly by Ni3Nb type γ″ and partially by Ni3Al type γ′ precipitation is today’s most widely used
superalloy in the world. China has paid special attention on Inconel 718 research and development.
Systematic long-term research project has been conducted in close cooperation among our university,
research institutes and factories. The goal of this long-term project is in 2 steps. First step is to
improve Alloy 718 to get high quality and the second step is for improving the alloy temperature
capability from 650oC to 680-700oC.
The basic idea for alloy improvement is still to keep the chemical composition in the range of
specification by small adjustment or control of minor elements, such as S, P, N, Si and Mg. The main
achievements are segregation control by adjustment of S, P, Si, control of N for cleanliness,
micro-alloying of Mg for grain boundary strengthening and control of low S and high P for stress
rupture life improvement.
Modification of Alloy 718 is based on structure stability study and its improvement in adjustment of
main second phase strengthening alloy elements Nb, Ti and Al in total amount and the ratios among
them. The goal is to achieve more stable second phase strengthening by control of alloy elements.
Experimental results show that the future of modified 718 alloys is very attractive to raise the
temperature capability improvement from 650oC to 680-700oC.

Abstract: The microstructure and creep properties of two powder metallurgy (PM) ‘beta gamma’ titanium aluminide alloys are presented. Alloy powders with nominal compositions of TiAl-4Nb-3Mn (G1) and TiAl-2Nb-2Mo (G2) were produced by gas atomization and consolidated by a two-step hot isostatic pressing (HIP) process (1250 °C/200 MPa/1 hour + 1100 °C/200 MPa/3 hours + slow cooling to room temperature). After HIP, the materials were given a step cooled heat treatment (SCHT) of 40 min at 1400 °C, furnace cooling to 1280 °C, and air cooling to room temperature. Selected specimens were aged at 900 °C for 6 or 24 hours. The SCHT yielded similar fully lamellar microstructures for both alloys, with a lamellar spacing of 0.04 m, but with different grain sizes averaging 80 m (G1) and 40 m (G2). The aging treatments generated  precipitates along lamellar colony boundaries in both alloys, but along lamellar interfaces only in alloy G2. Constant load tensile creep tests were performed at 760 °C and 276 MPa. Alloy G2 exhibited superior creep performance compared to alloy G1, due to the quantity and size of  precipitate particles at the lamellar interfaces.

Abstract: The effect of Cd and Sb addition on the microstructural and mechanical properties of as-cast AZ31 alloys was investigated and compared. The results indicate that the difference of Sb and Cd in the microstructure and mechanical properties of as-cast AZ31 magnesium alloy is significant. Addition of 0.15%Sb (mass fraction) to AZ31 alloy can refine the matrix and β-Mg17Al12 phase but not form a new phase Mg3Sb2. Oppositely, by addition of 0.3-0.7% Cd to AZ31 alloy, Cd was dissolved into the AZ31 alloy, the phase composition did not change but was refined also. Accordingly, the Cd-refined AZ31 alloy exhibits higher tensile and impact toughness and Brinell hardness properties than the Sb- refined one. The difference of Sb and Cd in the mechanical properties is possibly related to the solid solution of Cd into the matrix and formation of Mg3Sb2 which has the same close-packed hexagonal structure as α-Mg.

Abstract: Bulk nanostructured (grain sizes in the range of 50-200nm) and ultrafine structured (grain sizes in the range of 100-500nm) -TiAl based alloys with compositions Ti-47Al (in at%) and Ti–45Al–2Cr–2Nb–1B–0.5Ta (in at%), respectively, have been produced using a combination of high energy mechanical milling of mixtures of elemental powders and hot isostatic pressing at 800 and 1000oC respectively, and the microstructures of the samples have been characterised. At room temperature, the HIPed samples fractured prematurely at tensile stresses in the range of 200-300MPa and showed no ductility, very likely due to the relative high oxygen content (0.6wt%) in the samples and very low tolerance of TiAl based alloys on dissolved oxygen. At 800oC, the HIPed samples showed a yield strength in the range of 55-70MPa, a tensile strength in the range of 60-80MPa, a large amount of elongation to fracturing around 100% and clear strain softening. Examination of the fractured tensile test specimens at room temperature and 800oC showed that the level of the consolidation was fairly high, but the HIPed samples do contain a small fraction of interparticle boundaries with weak atomic bonding. The fracture of the HIPed samples in tensile testing at room temperature and 800oC, respectively, is predominately intergranular, and the large amount of plastic deformation prior to fracture at 800oC is achieved mainly through grain boundary sliding in conjunction with dislocation gliding, in agreement with the deformation mechanisms of nanostructured and ultrafine structured alloys generally agreed by researchers.